Recovery of tetraalkyl lead from electrolytic reaction mixtures



Jan. 5, 1965 J. LlNsK ETAL RECOVERY OF TETRAALKYL. LEAD FROMELECTROLYTIC REACTION MIXTURES Filed Dec. so. 1960 United States Patentlliee 3,164,537 Patented Jan. 5, 1965 This invention relates toorganornetallic compounds, and more particularly concerns an improvedelectrolyti process for making tetraalkyl lead compound.

Tetraalkyl lead compounds such as tetraethyl and tetramethyl lead arecommercially important organometallic compounds. It has heretofore beendiscovered that tetraalkyl lead compounds such as these may be preparedby electrolyzing the corresponding alkyl Grignardreagent, c g., ethylmagnesium chloride, using a lead anode. By this process, alkyl groups inthe lGrignard reagent are hydrocarbon is withdrawn; this is completelydry and is suitable for recycle to alkyl Grignard preparation. Thebottoms from this distillation, containing tetraalkyl lead products,unreacted Grignard reagent, magnesium halide by-product, and highboiling ether, is hydrolyzed with v ether bottoms.

transferred to the anode, forming tetraalkyl lead and giving magnesiumchloride as a by-product. This electrolytic process is materiallysuperior to the conventional sodium-lead process by reason of its lowercapital, raw material, and operating expenses.

It has also been discovered that the solvent used in the electrolysiscontrols, to a large extent, the practical success of the process. Atpresent, a mixture of certain high boiling ethers with lower boilingtetrahydrofuran, a normally liquid aromatic hydrocarbon such as benzene,and excess alkyl halide constitute the solvent of choice. Grignardreagents in this mixture possess high electrical conductivity, whichthus reduces 12R losses and makes for high overall electricalefficiency. In addition, the solvent mixture maintains its fluidity andsolvent capacity for the various reactants and products during allstages of the electrolysis. Moreover, the presence of excess alkylhalide avoids undesirable gassing and provides an electrical eiciencysubstantially in excess of the theoretical four Faradays per mol ofproduct.

However, by its very complexity, mixed alkylhalideether-tetrahydrofuran-aromatic solvents' present diiculties withrespect to recovering thetetraalkyl lead product and the various solventcomponents in a form suitable for reuse. Distillation alone, forexample, is not satisfactory inasmuch as the high temperatures necessaryto separate tetraalykyl lead from high boiling ethers result in chemicalattack of the expensive ethers by residual Grignard reagent. Priordecomposition of the Grignard reagent by hydrolysis is not always thesolution since, in that case, overhead products will Abe contaminatedwith water. Accordingly, the major object of this invention is toprovide a process for recovering tetraalkyl lead and solvent componentsfrom the aforementioned electrolytic reaction by a procedure whichcombines the simplicity of distillation with thesolvent-conservingfeatures of hydrolysis. At the same time, Grignardattack on the ether is virtually completely prevented, and most of thesolvent can be recovered in adry form suitable for recycle. t

. In accordance with the invention, tetraalkyl lead is firstmanufactured by now-conventional electrolytic procedures, utilizing alead anode and a solution of alkyl Grignard reagent in a solventcomprising excess alkyl halide, a high boiling'ether, tetrahydrofuran,and an aromatic hydrocarbon. After electrolysis has proceeded to anextent where a substantial amount of alkyl Grignard reagent is convertedto tetraalkyl lead, the electrolytic reaction mixture is subjected to arst .low-temperature distillation. To avoid mechanical problems it` maybe advantageous to recycle a portion of the dry, high boiling ether-tothis distillation step. The distillation overhead, containing alkylhalide, tetrahydrofuran, and aromatic aqueous hydrochloric acid todecompose the Grignard.

Hydrolysis produces two liquid phases. The aqueous phase is chieflycomposed of magnesium halide, while the organic phase containstetraalkyl lead and high boiling ether, possibly with al portion ofunremoved aromatic hydrocarbon.

This organic phase is then redistilled either at subatmospheric pressureor in the presence of steam to separate a tetraalkyl lead overhead fromthe high boiling If this distillation employs steam, the ether may bedried by redistillation and then returned to electrolysis, while ifsubatmospheric pressure distillation is employed the bottoms per se aresuitable for recycle. The overhead from this distillation is tetraalkyllead product, possibly containing a small amount of aromatichydrocarbon, and may then be Withdrawn as the nal product of theinventive process.

The invention in its various aspects will be more fully described andmore clearly understood from the ensuing specification, which is to beread in conjunction with the attached single drawing showing, inschematic form, a owsheet exemplifying the principles of this invention.For sake of clarity and simplicity of presentation, many auxiliaries andutilities have been omitted from the drawing, but those skilled in theart will readily perceive the need for and appreciate the location ofsuch items as pumps, gauges, duplicate items of equipment, distillationtower auxiliaries, etc.

Before discussing the owsheet however, it is of advantage to clarifycertain aspects of the electrolytic process.. Essentially, two mols ofalkyl Grignard reagent, two mols of alkyl halide, and 1 of Ametalliclead are converted to 1 mol of tetraalkyl lead and two mols of byproductmagnesium dihalide, the electrolysis requires a solvent for the Grignardreagent, and this is optimally a solution of excess alkyl halide, a highboiling ether, tetrahydrofuran, and a normally liquid aromatichydrocarbon.

The high boilingether'may be any ether boiling above the boiling pointof the particular tetraalkyl lead compound being manufactured. Of themany ethers which may be used, thosewhich are dialkyl ethers of anethylene glycol, having at least two carbon atoms in each alkyl groupand notmore than two ethylene groups in the glycol portion, arepreferred, since the resultant electrolyte is able to preserve itshomogeneity over a wide range of concentrations and temperatures. Thus,exemplary glycol ethers are the diethyl ether of ethylene glycol, thedibutyl ether of ethylene glycol, the`diethyl ether of dethylene glycol,the dibutyl ether of diethylene glycol, and the hexylethyl ether ofdiethylene glycol. Ethers other than those within the foregoingpreferred class may also be used, and as examples of these there may bementioned the dimethyl ether of diethylene glycol, the dimethyl ether oftriethylene glycol, and various cyclic ethers, keeping inmind that theether should boil higher than the tetraalkyl lead product (tetramethyllead boils at C. and tetraethyl lead boils at about 195 Cwithdecomposition). t t

Aromatic hydrocarbons as'components of the electrolyte may be any of thearomatics which are liquidat room temperature and which are inert toGrignard reagents under conditions of the electrolysis. Benzene and thealkyl benzenes such as toluene, the xylenes, cumene, the cymenes, etc.may be used, although it has been found that the simpler alkyl benzeneshaving only methyl groups offer some advantages with respect to lowboiling points and high stability.

The overall electrolyte composition may be varied over wide ranges,depending upon desiderata of chemistry and economics. Y The alkylGrignard reagent is advantageously employed at a concentration withinthe range of about 1.5 to about 3.5 Normal, preferably within the rangeof about A2'to about 2.5 Normal at the start of electrolysis. Enoughhigh boiling ether and tetrahydrouran (which, of course, is an ether)should be used so that approximately one mol lof ether is available foreach molecule of magnesium chloride formed in the. electrolysis. Highboiling ether may thus be employed at a concentration of from about l toabout 90%, more or Y less, Vby weightof solution, while tetrahydrofuranmay suitably be used at a concentration between about 2 and 60% or moreby weight. The aromatic hydrocarbon can vary over a broad range, with aslittle as 2 or 3 weight percent offering' substantial advantages whichare even further increased byusing concentrations of as high as 50weight percent or more. .Alkyl halide concentrations of, say, 1-50weight percent-tof total electrolyte should also be used, since excessalkyl halide materially improves electrolyte conductivity vand reducesthe formation of byproduct ga'ses. t

It appears that the optimum electrolyte composition prior toelectrolysis is an alkyl Grignard concentration of about 1.5-3.5 Normal,a high boiling ether concentration of about 40-80%, about 10-40%tetrahydrofuran, abouty 30-50weight percent aromatic,"and about 1-10weight percent excess alkyl halide. Y

The alkylGrignard reagents are chosen to produce the desired tetraalkyllead product. For example, an ethyl Grignard'will afford tetraethyllead, while'a methyl Grignard will yield tetramethyl lead. Alkyl leadcompounds having from 1 to about 4 ycarbon atoms per alkyl group aremost effectively produced by the electrolytic process. By employingmixed Grignard reagents, such as a mixture of ethyl Grignard and methylGrignard, or a mixture of ethyl Grignard with methyl halide, a mixedtetraalkyl lead compound may be'prepared; thus, dimethyl diethyl leadcan be produced readily. The halide portion of the alkyl Grignardreagent may be the chloride, bromide, or less, desirably, the iodide'.Conditions in a typical electrolytic cell advantageously include atemperature Within the range of about 20 C. to about 100 C., preferablyabout 20450o C., and optimally about 25-35 C. Acurrent density, at boththe anode and cathode, withinV the range of about 0.2 to about 100amperes per lsquare foot is advantageously employed. Relatively lowvoltages, of the order of about 20-30 volts, are preferred, althoughcell voltages of 50 volts and even higher maybe utilized. Cell pressuresmay range from atmospheric Vto low superatmospheric, say 60 p.s.i.g.,but are preferably lower, i.e. less than about 30 p.s.i.g.Theelectrolysis is advantageously continued, in either a batchwise orcontinuous process, until a substantial amount of the Grignard reagentis converted to tetraalkyl leadandadvantage'ously is conducted until atleast a major amount, and preferably at least about 80% of the GrignardreagentV is so converted.

Turning now to the drawing,'Gregnard reagent is prepared in preparationzone 11 for Ythe ultimate preparation of tetramethyl lead. Methylchloride -is admitted from source 12 via line V13, while'magnesiumVmetal chips are chargedfrorn symbolic source 14 and line 16 to thepreparation zone. Recycleether, in this case hexylethyl ether ofkdiethylene glycol, isrecycled via line 17, while recycle methyl`chloride and tetrahydrofuran are introduced through line 18. Make-up.solvent components, eg., benzene, .tetrahydrofuram and ether, arecharged as needed through line 19. Grignard solution from preparationzone 11 is 1.98 molar with respect to methyl Grignard and has anapproximate composition of 3638 parts by weight of alkyl Grignardreagent in hexylethyl ether of diethylene glycol, 1307 parts of benzene,131 parts of tetrahydrofuran,and 103 parts ofrmethyl chloride. (An

4 additional 347 parts of methyl chlorideis added to cell 22 during therun.) It is charged via line 21 to electrolytic cell 22, which isprovided with lead anodes 23 and stainless steel cathodes 24 connectedto a direct current power supply 26. Electrolysis is commenced andtemperature is maintained at 36-41 C. by recirculation of electrolytethrough an external heat exchanger, not shown. Electrolysis is commencedwith an initial plate voltage of 17.5 and an amperage of 19.0, whichvoltage is progressively increased during the run to a maximum of 40.4volts at the termination, l1 hours. The average voltage is 24.9, whilethe average temperature is 41.1 C. and pressure is 3.1 p.s.i.g. Grignardconversion is 95.3%, of which 99.1% is traceable to tetramethyl lead.The current eiliciency is 158% of theoreticahand the power required is2.86 kilowatt hours per pound of tetramethyl lead.

After the 11 hours of electrolysis, the reacton mixture is withdrawn vialine 27 and .conducted to subatmospheric pressure distillation towerZS.This tower, advantageously operating at a pressure below about 500millimeters mercury absolute and a 'pot temperature below about C., andhaving, say, 5-20 theoretical plates and aV reflux ratio of about 5-20reux-to-product, separates an overhead containing substantially all ofthe unreacted excess' alkyl halide; substantially all of thetetrahydrofuran, and at least a major portion of the benzene from abottoms containing substantiallyall of the tetramethyl lead, unreactedGrignard reagent, magnesium chloride,` a minor amount of the benzene,and hexylethyl ether of diethylene glycol. Tower 28 can easily beoperated so that the overhead contains less'than 3% by weight of thetetramethyl lead production while the bottoms contains less than aboutV0.5% of the' tetrahydrofur'an.

In tower 28 the overhead vapors pass via vapor line 29 to condenser 31,where substantially all of the overhead components other than a portionof the methyl chloride and by-product methane gas areV condensed. Fromcondenser 31V the condensate and Uncondensed gas pass via line 32 tooverhead receiver 33, which supplies reflux to tower 28 via line 34, andrecycle dry solvent to Grignard kpreparation tank 11 Vialine 36.Uncondensed material in overhead receiver 33 passes fvia vapor line 37to refrigeration condenser 38 where methyl chloride is condensed andreturned to receiver 33, while methane and other noncondensible gasesare exhausted via line 39 to the atmosphere. f

The bottoms fromV tower28 contain more than 97% of the tetramethyl leadtogether with unreacted Grignard reagent, magnesium halide, less'thanabout half of the benzene, and hexylethyl ether of diethylene glycol.VThis stream is transferred via line 41 to hydrolyzing tank 42, equippedwith a stirrer 43 or other equivalent agitating device. Here a stream ofaqueous hydrochloric acid, e.g., 3% by weight, is admitted via source 44and line 46; The amount of hydrochloric acid added is stoichiometricallyequivalent to the unreacted Grignard reagent, while the amount of wateris at least that necessary-to dissolve magnesium chloride formed bydecomposition of the Grignard and that initially present as an etheratecomplex in the electrolytic reaction mixture.

From hydrolysistank V42jthe resultant mixture of aqueous magnesiumchloride-containing phase and organic phase is pumped via line 47 tosettling drum 48. Here the heavier aqueous phase is withdrawn viapline49 as a bottoms material, while the 'organic phase is'taken overhead vialine V51. The'raqueous phase contains-substantially less than a percentor so each of hexylethyl ether of diethylene glycol, benzene, andtetramethyl lead, and may be disposed of, advantageously afterdecomposition of the lead by chlorination. ,Y The organic phase containssubstantially all (except minor losses) of the tetramethyl lead,together with hexylethyl ether of diethylene glycol and aromatic hydro-3,1 ease? carbon. It is conducted via line 51 to distillation tower Inthe embodiment depicted in the drawing, distillation tower 52 issupplied with steam to effect separation of the tetramethyl lead (andbenzene) from hexylethyl ether. Tower 52 may have, say, 3-8 theoreticalplates and may operate at a pressure not substantially aboveatmospheric, e.g., 15 p.s,i.a., with reboiler temperature below about110 C. and a reux ratio of between about 1 and 5. Steam is admitted nearthe bottom via line 53, while the steam-tetramethyl lead-benzeneoverhead is passed via vapor line 54, condenser 56, and condensate line57 to overhead receiver 58. The aqueous layer, consisting essentially ofwater with a small amount of dissolved benzene and tetramethyl lead, istaken from the upper portion of overhead receiver 58 and relluxed to thetower 52 via line 59, with the excess being discarded via line 61. Steammay be employed in amounts of between about 0.1 and l mols/mol feed.

The product of the inventive process is recovered via line 62. It iscomposed of tetramethyl lead, together with some-usually a minoramountof benzene and a small amount of dissolved and/or entrained water.It may be dried before blending into motor or aviation mix according towell known techniques and recipes.

The bottoms from distillation tower 52 is composed almost entirely ofthe high boiling hexylethyl ether of diethylene glycol, with a smallamount of water and tetramethyl lead being present. It is taken olf vialine 63, sent to settling drum 64 Where the aqueous phase is removed vialine 66 and the bulk of ether is transported via line 67 to drying tower68. Tower 68, having, say, 1-5 theoretical plates and a reux ratio inthe range of about 1-5, and operating at a pressure of not substantiallyabove atmospheric, e.g., 14.5 p.s.i.a. with the reboiler temperature ofup to about 130 C., effectuates drying of the ether by fractionaldistillation. Water is taken overhead via line 69 (through aconventional reux system, not shown), while the dried ether is conductedvia line 71 to the electrolysis zone via Grignard prep-aration zone 11and, optionally, via a solid desiccant 72 such as absorbent alumina or amolecular sieve.

As an alternate to the use of steam, tower 52 may be operated undervacuum in order to maintain the reboiler temperature below the desiredmaximum of about 110 C. For this operation, a subatmospheric pressureof, say, 300 millimeters mercury absolute is imposed, and the overheadwill consist of tetramethyl lead and benzene, while the bottoms willconsist of hexylethyl ether of diethylene glycol with possibly a smallamount of tetramethyl lead. For Vacuum operation, overhead receiver 58will have only a single phase therein, and accordingly aqueous productline 61 may be dispensed with. Similarly, there is no need for settlingdrum 64, drying tower 68, or desiccant 72, since the ether is recoveredin a dry form, suitable for recycle to the electrolysis, and via line 70to the first distillation in tower 28.

The iinal tetrarnethyl lead product is of high quality,

exemplary description. It is intended to embrace all such alternatives,modifications and variations as fall within the spirit and broad scopeof the appended claims.

We claim:

1. A process `for recovering a tetraalkyl lead product and solvent fromthe electrolytic reaction mixture obtained by electrolyzing an alkylGrignard reagent with a lead anode in a solvent comprising excess alkylhalide, a high boiling ether, tetrahydrofuran, and an aromatichydrocarbon, which process comprises: distilling said mixture to obtainan overhead containing excess alkyl halide, tetrahydrofuran, andaromatic hydrocarbon, together with a bottoms containing tetraalkyl leadproduct, unreacted Grignard reagent, magnesium halide, and said highboiling ether; recovering said overhead; hydrolyzing the bottoms withaqueous hydrochloric acid; separating the resulting aqueous phasecontaining magnesium halide from an organic phase containing tetraalkyllead product and said high boiling ether; and distilling said organicphase to separate tetraalkyl lead product as an overhead from the highboiling ether as bottoms.

2. Process of claim l wherein distillation of said electrolytic reactionmixture is effected at a pressure of below about 500 millimeters mercuryabsolute and at a temperature of below about C.

3. Process of claim 1 wherein distillation of said organic phase iseffected at subatmospheric pressure.

4. Process of claim 1 wherein distillation of said organic phase iseifected in the presence of steam, the resultant overhead is separatedfrom water, and the resultant high boiling ether bottoms are redistilledto eiect drying thereof. y

5. Process of claim l wherein said tetraalkyl lead product istetramethyl lead.

6. Process of claim 1 wherein said tetraalkyl lead product is tetraethyllead.

7. A process for recovering a tetraalkyl lead product and solvent fromthe electrolytic reaction mixture obtained by electrolyzing thecorresponding alkyl Grignard reagent with a lead anode in a solventcomprising an exfcess of the corresponding alkyl halide, a dialkyl etherof an ethylene glycol having at least two carbon atomsin each alkylgroup and not more than two ethylene groups in the glycol portion,tetrahydrofuran, and an aromatic hydrocarbon, which process comprises:distilling said mixture to obtain a dry overhead containingsubstantially all of the unreacted excess alkyl halide andtetrahydrofuran and at least a major portion of the aromatichydrocarbon, together with a bottoms containing substantially all of thetetraalkyl lead product, unreacted Grignard reagent, magnesium halide, aminor portion of the aromatic hydrocarbon, and ether, recovering saidoverhead; hydrolyzing the bottoms with suicient aqueous hydrochloricacid to dissolve the magnesium halide; separating the resulting aqueousphase containing magnesium halide from an organic phase containingtetraalkyl lead product, aromatic hydrocarbon, and ether; and distillingsaid organic phase to separate tetraalkyl lead and atleast a portion ofthe aromatic hydrocarbon as an overhead from the ether bottoms.

References Cited in the tile of this patent UNITED STATES PATENTS2,535,190 Calingaert Dec. 26, 1950 2,777,867 Giraitis et al 1 Ian. 15,1957 2,944,948 Giraitis et al July 12, 1960 2,985,568 Ziegler et al. May23, 1961 3,007,858 Braithwaite Nov. 7, 1961 3,028,319 Kobetz et al Apr.3, 1962

1. A PROCESS FOR RECOVERING A TETRAALKYL LEAD PRODUCT AND SOLVENT FROMTHE ELECTROLYTIC REACTION MIXTURER OBTAINED BY ELECTROLYZING AN ALKYLGRINGNARD REAGENT WITH A LEAD ANODE IN A SOLVENT COMPRISING EXCESS ALKYLHALIDE, A HIGH BOILING ETHER, TETRAHYDROFURAN, AND AN AROMATICHYDROCARBON, WHICH PROCESS COMPRISES: DISTILLING SAID MIXTURE TO OBTAINAN OVERHEAD CONTAINING EXCESS ALKYL HALIDE, TETRAHYDROFURAN, ANDAROMATIC HYDROCARBON, TOGETHER WITH A BOTTOMS CONTAINING TETRAALKYL LEADPRODUCT, UNREACTED GRIGNARD REAGENT, MAGNESIUM HALIDE, AND SAID HIGHBOILING ETHER; RECOVERING SAID OVERHEAD; HYDROLYZING THE BOTTONS WITHAQUEOUS HYDROCHLORIC ACID; SEPARATING THE RESULTING AQUEOUS PHASECONTAINING MAGNESIUM HALIDE FROM AN ORGANIC PHASE CONTAINING TETRAALKYLLEAD PRODUCT AND SAID HIGH BOILING ETHER; AND DISTILLING SAID ORGANICPHASE TO SEPARATE TETRAALKYL LEAD PRODUCT AS AN OVERHEAD FROM THE HIGHBOILING ETHER AS BOTTOMS.